1. Field of the Invention
The present invention relates to an oxygen adsorbent, to a method for producing the oxygen adsorbent, and to an exhaust gas purifying catalyst that utilizes the adsorbent. More specifically, the present invention is directed to an oxygen adsorbent that contains a ceria composite oxide, that has an oxygen adsorption capacity and a S (sulfur) poisoning resistance and that can prevent deterioration of a catalyst, to a method for producing the oxygen adsorbent, and to an exhaust gas purifying catalyst that contains the oxygen adsorbent.
2. Description of the Related Art
Exhaust gas discharged from an internal combustion contains HC (hydrocarbons), CO (carbon monoxide) and NOx (nitrogen oxides), and these pollutants are emitted into the atmosphere after being purified by an exhaust gas purifying catalyst. As a typical example of the exhaust gas purifying catalyst that is used for the purpose, a three-way catalyst that is composed of a porous oxide carrier such as Al2O3 (aluminum oxide), SiO2 (silicon oxide), ZrO2 (zirconium oxide) or TiO2 (titanium oxide) and a precious metal, such as Pt (platinum), Rh (rhodium) or Pd (palladium), that is supported on the carrier is widely used.
The three-way catalyst oxidizes HC and CO in exhaust gas and also reduces NOx and can exhibit the highest effect in the purification of an exhaust gas of a stoichiometric gas atmosphere that has been generated by burning at nearly the theoretical air-fuel ratio. However, the exhaust gas atmosphere shifts to the rich or lean side because the actual air-fuel ratio shifts to the rich or lean side with respect to the stoichiometric air-fuel (A/F) ratio depending on the operating conditions of the automobile. Thus, a high purification performance cannot be necessarily only obtained with a three-way catalyst that is configured as described above. In particular, because requirements for improved fuel efficiency have become more common in recent years, there is an increasing opportunity for an exhaust gas purifying catalyst to be exposed to sudden fluctuations of atmosphere due to fluctuations of the air-fuel ratio under high temperatures that are caused by, for example, an increase of the number of times of fuel cut (FC) at a high temperature. Such sudden fluctuations of atmosphere can significantly accelerate deterioration of the catalyst.
Thus, to enhance the exhaust gas purification ability of a three-way catalyst by compensating for fluctuations in oxygen concentration of the exhaust gas, an oxygen adsorbent that occludes oxygen when the oxygen concentration in the exhaust gas is high and releases oxygen when the oxygen concentration in the exhaust gas is low, is used in exhaust gas purifying catalysts. Well-known examples of the oxygen adsorbent include ceria (CeO2) and ceria composite oxide, that is, CeO2—ZrO2 composite oxide. Such oxygen adsorbents are an important constituent material for an automobile exhaust gas purifying catalyst to compensate for fluctuations in A/F ratio and to maintain an atmosphere in which the catalyst can act in the most effective manner. In order to purify the exhaust gas more stably, an oxygen adsorbent that adsorbs a larger amount of oxygen is desired. This is the reason why a lanthanoide oxysulfate (Ln2O2SO4), which can in theory reversibly adsorb eight times the amount of oxygen per molecule than CeO2 or a ceria composite oxide is attracting attention. However, because exhaust gas from the engine of an automobile contains sulfur components, such as sulfur oxides that includes SO2 (sulfur dioxide) and SO3 (sulfur trioxide), the oxygen adsorbent must be resistant to sulfur-poisoning (S-poisoning).
Japanese Patent Application Publication No. 2005-87891 (JP-A-2005-87891) describes a purifying catalyst that is provided with a perovskite-type composite oxide, represented by the formula AxB2−xCO4−y(SO4)y (where A represents a rare earth element, B represents an alkaline-earth metal, C represents a transition metal, 1≦x<2 and 0<y≦1), that is immune to S-poisoning. Japanese Patent Application Publication No. 2005-87892 (JP-A-2005-87892) describes a catalyst that includes an oxygen adsorbent composed of an oxysulfate of a rare earth, represented by A2O2SO4 (where A represents a rare earth element), and a precious metal that is supported on the oxygen adsorbent. The catalyst described in JP-A-2005-87892 has a higher oxygen adsorption capacity than conventional catalysts composed of a CeO2—ZrO2 composite oxide on which a precious metal is supported. Japanese Patent Application Publication No. 2006-75716 (JP-A-2006-75716) describes an exhaust gas purifying catalyst, which includes a first oxygen adsorbent composed of a compound represented by A2O2SO4 (where A represents a rare earth element), and an NOx occluding material that supports a precious metal. The catalyst described in JP-A-2006-75716 has a high oxygen adsorption capacity under high temperature, and demonstrates the use of the first oxygen adsorbent in combination with a second oxygen adsorbent such as CeO2 or a CeO2—ZrO2 composite oxide. However, no specific example in which a CeO2—ZrO2 composite oxide is used as the second oxygen adsorbent is described. In addition, Japanese Patent Application Publication No. 2008-284512 (JP-A-2008-284512) describes an oxygen adsorbent that includes a compound composed of Pr2O2SO4 and/or Pr2O2S in which at least a portion of the Pr (praseodymium) is substituted by Ce (cerium), and an exhaust gas purifying catalyst that includes the oxygen adsorbent and a metal that is supported on the oxygen adsorbent.
However, the oxygen adsorbents that are described in these Patent Documents do not have a sufficient oxygen adsorption capacity, so that there is no alternative but to use a large amount of the catalyst. This is the reason why an oxygen adsorbent that has a higher oxygen adsorption capacity is desired.